Activity-dependent plasticity in the spinal dorsal horn may at least partly underlie the development of neuropathic pain following peripheral nerve injury. This form of plasticity allows for enhanced neuronal communication, and in the brain, it may enable learning and memory. However, in the spinal dorsal horn it may transform the essential but rapidly terminated sensation of acute pain into unproductive persistent pain. In this application we propose to employ the well-established loose ligation of the sciatic nerve model of neuropathic pain, and investigate the potential interaction between brain derived neurotrophic factor (BDNF) and the potassium-chloride co-transporter 2 (KCC2) in the early phase of injury-elicited spinal plasticity,i.e., in the first few hours after sciatic ligation. Our central hypothesis is that BDNF-mediated down-regulation of KCC2 promotes an early increase in dorsal horn neuronal excitability. We surmise that this increase in excitability contributes significantly to the initial development of spinal plasticity and to the ensuing early behavioral signs of neuropathic pain. In Specific Aim 1 we will examine the early consequences of sciatic ligation on KCC2 gene expression and protein levels in the dorsal horn, and establish if changes in KCC2 levels are dependent on the activation of the high-affinity BDNF TrkB receptors. In Specific Aim 2 we will functionally assess the early consequences of sciatic ligation or local BDNF application on neuronal excitability, and we will determine if a block of TrkB receptors or KCC2 activity modifies this excitability. In Specific Aim 3 we will establish the behavioral relevance of the pharmacological manipulation of TrkB receptors or KCC2 activity on the early expression of neuropathic pain. Neuropathic pain is a serious health problem with profound effects on the quality of life. It is a debilitating disease that affects millions of people and costs billions of dollars in health care and lost productivity. Through a detailed examination of the interplay between BDNF and KCC2 we hope to achieve two goals. First, from a purely biological perspective we wish to delineate the early consequences of peripheral nerve injury on dorsal horn neuronal activity. Second, from a therapeutic perspective, we seek to achieve better target specificity for more effective future therapeutic interventions.